1. Field of the Invention
The present invention generally relates to amplitude modulation radiofrequency transmissions and, more specifically, transmissions performed with a modulation factor smaller than one.
An example of application of the present invention relates to electromagnetic transponder systems in which a high-frequency carrier is modulated at least in amplitude by a terminal for transmission to an electromagnetic transponder, for example, carried by a smart card, in the field of the terminal.
2. Discussion of the Related Art
Electromagnetic transponder systems are based on the cooperation between an oscillating circuit on the read/write terminal side and a resonant circuit on the electromagnetic transponder side (generally, a portable element), to exchange information by using a high-frequency field radiated by the oscillating circuit of the terminal. Most often, the high-frequency carrier is also used as a remote-supply carrier providing the transponder supply power.
Such systems are for example described in U.S. Pat. Nos. 6,031,319 and 6,703,921, which are incorporated herein by reference.
An example of application of the present invention relates to transponder systems based on standards ISO 14443 and 15693 according to which the remote-supply carrier radiated by the terminal is 13.56 MHz, while a back-modulation sub-carrier may be used by the transponders to transmit information to the terminal with a 847.5-kHz frequency. This sub-carrier is however not always present, according to applications (of these standards or not). In the terminal-to-transponder direction, the carrier is modulated in amplitude with a modulation factor generally on the order of 10% with a flow rate on the order of 106 kbits/second. The modulation factor is defined as being the amplitude difference between the transmission of a state 1 and the transmission of a state 0, divided by the sum of these amplitudes.
In fact, the standards set a range of acceptable modulation factors that the transponders are supposed to be able to interpret, and that the terminals are supposed to comply with. This range is, in the case of the above-mentioned standards, between 8 and 14%.
FIG. 1 very schematically shows, in the form of blocks, an example of an electromagnetic transponder system to which the present invention applies. A transponder 1 (TR) is intended to be placed in the electromagnetic field of a terminal 2 (TERM) having an inductive element L2 of an oscillating circuit emitting a high-frequency radiation detected by an antenna L1 of transponder 1.
FIG. 2 very schematically shows, partly in the form of blocks, an example of a conventional architecture of an electromagnetic transponder 1, intended to communicate with a read/write terminal (not shown in FIG. 2). The transponder comprises an oscillating circuit 10, formed of an inductive element L1 forming an antenna, in parallel with a capacitor C1 at the A.C. input terminals of a rectifying bridge 11. The rectified output terminals of bridge 11 are connected by a storage capacitor Cs.
The signal detected when transponder 1 is in the field of a terminal is used, on the one hand, for extracting a supply voltage Vdd from the transponder circuits, generally by means of a regulator 12 (REG) and, on the other hand, for decoding the possible data transmitted by the terminal. For this purpose, the transponder includes an amplitude demodulation circuit including, for example a filter formed of a resistor R5 connecting the output of bridge 11 to a first electrode of a capacitor C3 having its other electrode connected to ground (second output terminal of bridge 1); a capacitor C4 conveying the A.C. output component of the previous filter; a resistor R6 connecting the electrode of capacitor C4 opposite to the filter to a first electrode of a capacitor C5 having its other electrode connected to ground; two comparators 13 and 14 of the signal sampled from the second electrode of capacitor C4 with two thresholds Vdd/2−ref1 and Vdd/2+ref1; and an RS-type flip-flop 15, having its set and reset inputs S and R receiving the respective outputs of comparators 13 and 14 and having its output D providing the detected (demodulated) state to a digital interpretation circuit 16 (for example, an arithmetical and logic unit UART).
Thresholds Vdd/2−ref1 and Vdd/2+ref1 are set by a dividing bridge formed of four resistive components R1, R2, R3, and R4 in series between two terminals of application of voltage Vdd, resistors R1 and R4 being of same value and resistors R2 and R3 being of same value so that the bridge sets, via resistor R6, a voltage proportional to half Vdd/2 of supply voltage Vdd.
The function of resistor element R6 is to transfer, onto the sampling node of the signal to be interpreted, value Vdd/2 to center the variations due to the state switching edges of the envelope of the amplitude-modulated signal.
A demodulator such as shown in FIG. 2 is described, for example, in U.S. Pat. No. 6,031,319.
To simplify the representation of FIG. 2, account has only been taken of the receive portion of the transponder. In particular, the back-modulation elements enabling modifying the load formed by the transponder in the electromagnetic field of a terminal for a transmission in the terminal-to-transponder direction have not been shown.
FIGS. 3A, 3B, 3C, and 3D illustrate the operation of the demodulator shown in FIG. 2. FIG. 3A shows an example of the shape of a signal transmitted by a terminal to transponder 1. FIG. 3B illustrates the shape of the signal received at the output of rectifying bridge 11 (upstream of filter 13). FIG. 3C illustrates the shape of the signal applied on the comparison inputs of comparators 13 and 14 (for example, the non-inverting input of comparator 13 and the inverting input of comparator 14) and the comparison thresholds set by resistors R1, R2, R3, and R4 (for example, Vdd/2+ref1 above half of voltage Vdd, and Vdd/2−ref1 under). FIG. 3D illustrates the result provided by the D output of flip-flop 15.
As illustrated in FIG. 3A, the carrier (for example, at 13.56 MHz) is modulated in amplitude with a modulation factor according to the state of the transmitted bit. The rate of the amplitude modulation is smaller than the carrier frequency. It can arbitrarily be considered that the high state (level a) corresponds to a state 1, while the low state (level b) corresponds to a state 0. The modulation factor (a−b/a+b) has been exaggerated in FIG. 3A. It is in practice smaller than 20% in ISO standard 14443, type B.
As illustrated in FIG. 3B, once rectified and filtered by elements R5 and C3, the signal is, in principle above or under its average value according to state 1 or 0 of the transmitted bit.
As illustrated in the left-hand portion of FIG. 3C, capacitor C4 filters the D.C. component so that the signal applied on the comparison inputs of comparators 13 and 14 only comprises, around value Vdd/2, edges at state switchings.
As illustrated in FIG. 3D, comparators 13 and 14 detect when the signal comes out of the window defined by thresholds Vdd/2−ref1 and Vdd/2+ref1, and flip-flop 15 provides a state 0 or 1 according to the direction of the detected edge.
However, if the terminal is not able to maintain a sufficient modulation factor (in practice, of at least some ten percent), interpretation errors under the effect of noise are possible, the intervals between the received levels and the thresholds becoming too small. In the extreme case, the thresholds may surround the received signal in both states 0 and 1.
This phenomenon is illustrated in the right-hand portions of FIGS. 3A to 3D which show the case of a signal between amplitudes a′ and b′ defining a smaller modulation factor than in the left-hand portion. As illustrated in FIGS. 3B and 3C, possible noises are likely to come out of the range defined by thresholds Vdd/2−ref1 and Vdd/2+ref1 whatever the state of the transmitted bit, generating erroneous detections (FIG. 3D).
The above problem is more and more present due to the multiplication of the types of terminals and of the types of transponders which are likely to cooperate with one another. Indeed, according to the manufacturing and the architecture of the terminal and/or of the transponder, their respective operation characteristics are likely to influence the other transmission element (transponder or terminal) in a different manner. Further, the environment in which the terminal and the transponder are present may also influence the electromagnetic field and the modulation factor effectively transmitted by the terminal.
In practice, the terminals are set in a characterization phase so that their modulation factor ranges, in the above-mentioned standards, between 8 and 14%. However, in real operation, this rate is likely to vary for the above-discussed reasons and to become undetectable by a transponder.
The fact that a transponder may not succeed in interpreting the data received by a terminal is a first disadvantage.
A second disadvantage is that the terminal receiving no response from the transponder knows that it has not been understood, but does not know why. In particular, other malfunctions may cause a poor detection of the signal by the transponder, without this being linked to the modulation factor.
It would be desirable for the terminal to be able to modify in real time the modulation factor of the signal that it transmits so that said signal can be properly detected by a transponder.
The setting of the modulation factor is not a practical difficulty since terminals are generally equipped with microprocessors to set the transceiver circuits and this setting capacitance most often already exists for the terminal characterization phase. However, terminals are currently unable to set this modulation factor in real time, since they have no coherent information about the reason which causes this absence of a proper reception by a transponder.
The notion of modulation factor is used to characterize the amplitude difference between amplitude-modulation transmitted states 0 and 1. However, other parameters such as, for example, the modulation index, which corresponds to the ratio (a/b) between the two amplitudes, are sometimes used. Referring to these parameters would amount to the same, and would only be other ways of discussing the problem.